Membrane for a pressure dome

ABSTRACT

The present invention relates to a membrane for a pressure dome suitable for cooperation with a pressure sensor. The membrane comprises a resilient circular wall suitable for closing one side of the pressure dome so as to define a partition between the inside of the pressure dome and the outside; and a circular rim suitable for being joined to a main body of the pressure dome. The membrane is characterized in that the resilient circular wall, when there is no difference between the pressures acting respectively on the inner surface and on the outer surface, has an outwardly convex form. The invention also relates to a pressure dome comprising the membrane.

The present invention relates to a chamber for measuring the pressureinside a pipe, in particular in an extra-corporeal circuit.

During therapeutic treatment requiring extra-corporeal circulation, asfor example in the case of hemodialysis, the pressure inside the circuitmust be monitored. For this purpose, the machine used for thetherapeutic treatment usually comprises suitably designed sensors. It isobviously necessary to prevent the fluids contained in the circuit fromcontaminating these sensors which are intended to be used repeatedly. Onthe other hand, the extra-corporeal circulation is performed in adisposable circuit.

For this purpose it is known to provide the circuit with at least onepressure chamber, usually called a “pressure dome”, suitable forcreating an interface between the circuit and the pressure sensor. Thepressure dome usually comprises a housing with an inlet and outlet whichare respectively connected to the circuit. Finally an elastomer membranecloses one side of the chamber and is formed so as to be able to makecontact with a pressure sensor. The elastomer membrane is highlyresilient so that it is able to transmit to the sensor the pressurepresent inside the circuit and the associated variations. A pressuredome of this type is schematically shown in FIG. 1 and is described indetail in the U.S. Pat. No. 7,603,907.

These pressure domes, although widely used, are however not defect-free.

A first defect consists in the fact that air may infiltrate between themembrane and the sensor. This phenomenon may occur, for example, whenthe sensor is joined to the pressure dome. During positioning of thesensor it is possible in fact for an air pocket to remain trappedbetween the two surfaces which instead should remain in direct contact.In such a case the sensor is no longer able to measure adequately thepressure of the chamber nor provide a reliable response with regard tovariations thereof.

This problem is further exacerbated should the circuit have internally anegative pressure, i.e. a pressure lower than atmospheric pressure. Insuch a case, which typically occurs upstream of the pumps located alongthe circuit, the membrane assumes a concave form, i.e. is “sucked”towards the inside of the pressure dome and worsen the precision ofmeasurement.

Furthermore there exists the problem of ageing of the elastomer fromwhich the membrane is made. This ageing results in the loss of flatnessof the membrane. It is clear that a membrane which has lost itselasticity and its flatness may easily result in the formation of airpockets between the pressure sensor and the membrane itself. It shouldbe remembered in this connection that the pressure domes and associatedmembranes are generally intended to have a shelf life of several yearsfrom time of production to actual use. This shelf life, which is quitereasonable from a logistical point of view, risks a loss of its designcharacteristics.

Another problem of the pressure dome of the known type is insteadassociated with the technology which is currently used to manufactureit. The main body of the chamber is made, in a manner known per se, bymeans of molding of a polymer which is sufficiently rigid and suitablefor contact with the physiological fluids. The elastomer membrane,instead, is made by means of two-component injection molding which isused to produce, in addition to the actual elastomer membrane, also afixing ring made with a rigid polymer, for example such as that used forthe main body. The elastomer membrane and the associated ring thus forma single part made of two different materials. They are then joined tothe main body, for example by means of a screw/female-thread,snap-engaging, interference or similar connection.

The manufacture of the pressure dome of the known type, which comprisesa two-component injection molding operation, therefore requires the useof molds with movable parts, the manufacture and use of which aresomewhat complex. Moreover these movable-part moulds require an initialoutlay which is distinctly greater than that of ordinary molds.

The object of the present invention is therefore to overcome at leastpartly the drawbacks mentioned above with reference to the prior art.

In particular, one task of the present invention is to provide amembrane for a pressure dome which is able to reduce to a minimum thepossibility of air entering into the joint with the pressure sensor.

Another task of the present invention is to provide a pressure domewhich may be made with simple and reliable technology, while stillensuring the high level of quality associated with the pressure domes ofthe known type.

The abovementioned object and tasks are achieved by a membrane accordingto Claim 1 and by a pressure dome according to Claim 7.

The characteristic features and further advantages of the invention willbecome clear from the description, provided hereinbelow, of a number ofexamples of embodiment provided purely by way of a non-limiting examplewith reference to the accompanying drawings.

FIG. 1 shows schematically a cross-sectioned side view of a pressuredome according to the prior art;

FIG. 2 shows schematically a cross-sectioned side view of a pressuredome according to the invention;

FIG. 3.a shows schematically a cross-sectioned side view of a membraneaccording to the invention;

FIG. 3.b shows schematically a cross-sectioned side view of anothermembrane according to the invention;

FIG. 4 shows schematically a detail of a cross-sectioned side view ofthe joint between the membrane and the pressure dome according toinvention;

FIG. 5 shows schematically three successive stages during assembly of apressure dome according to the invention;

FIG. 6 shows a pressure dome according to the invention combined with amulti-use protective element;

FIG. 7 shows schematically a cross-sectioned side view of anotherpressure dome according to the invention.

With reference to the accompanying figures, 10 denotes in its entirety apressure dome comprising a membrane 12.

The membrane 12 according to the invention comprises:

-   -   a resilient circular wall 120 suitable for closing one side of        the pressure dome 10 so as to define a partition between the        inside of the pressure dome and the outside;    -   a circular rim 124 suitable for being joined to a main body 16        of the pressure dome 10;        whereby the resilient circular wall 120, when there is no        difference between the pressures acting respectively on the        inner surface 121 and on the outer surface 122, has an outwardly        convex form.

Hereinbelow “inner” is understood here as meaning the part of thepressure dome 10 which, during use, is occupied by the physiologicalliquid. In relation to the membrane 12, therefore, the inner surface 121is that surface which, during use, is wetted by the physiologicalliquid.

The membrane 12 therefore is not flat, but is characterized by a doublecurvature. In other words, the resilient circular wall 120 assumes theform of a cap, for example a cap forming part of a sphere or other solidof rotation. In accordance with such a geometrical form, a maximumelevation f may be specifically defined for the resilient circular wall120. In this connection, see the diagrams in FIG. 3. This maximumelevation f is the distance between the outermost point of the cap andthe plane π containing the outer base circumference of the cap itself.

In accordance with certain embodiments of the invention, the cap definedby the resilient circular wall 120 has a maximum elevation of between 1%in 2% of the diameter d of the base circumference of the cap.

In accordance with the embodiment of the invention shown in FIG. 3, thecap defined by the resilient circular wall 120 has a maximum elevation fequal to about 1.7% of the diameter d of the base circumference of thecap. In particular, in this embodiment, the diameter d is equal to 17.7mm and the maximum elevation f is equal to 0.3 mm.

As already described above, the resilient circular wall 120 has anoutwardly convex form, provided that there is no difference between thepressures acting on the inner surface 121 and on the outer surface 122respectively. According to the embodiment of FIG. 3.a, both the innersurface 121 and the outer surface 122 have outwardly convex form. Insuch embodiment, the circular wall 120 has an almost uniform thickness.According to the embodiment of FIG. 3.b, while the outer surface 122 hasan outwardly convex form, the inner surface 121 is substantially flat.In such embodiment, the thickness of the circular wall 120 slightlyvaries along the radial direction, i.e. has a maximum in the centre ofthe wall 120 (i.e. the outermost point) and gradually reduces toward itsperiphery.

The membrane 12, as shown in FIG. 3, is preferably made as one piece. Inother words, the rim 124 is preferably formed integrally and as onepiece with the wall 120. Even more preferably, the rim 124 and wall 120are made by means of injection-molding of a single material. Forexample, the membrane 12 according to the invention may be made, in amanner known per se, using a thermoplastic elastomer or other elastomerssuitable for contact with the physiological fluids.

The circular rim 124 has a cross-section which is distinctly thickerthan that of the wall 120. With particular reference to FIG. 3, it canbe noted how the thickness of the circular rim 124 (measured in adirection substantially perpendicular to the plane π) is about threetimes that of the wall 120 (measured in the same way). This results, forthe same material used, in a greater rigidity of the rim 124 compared tothe rest of the membrane 12, in particular compared to the wall 120.

The relative rigidity of the rim 124 is necessary in order to be able tofix effectively and stably the membrane 12 to the main body 16 of thepressure dome 10. The joint between the membrane 12 and the main body 16is described in more detail below.

As can be clearly seen in FIG. 1, the resilient circular wall 120 of themembrane 12 according to the prior art is perfectly flat. In this way,the end surface 220 of the pressure sensor 22, which is also flat, mayideally rest on the resilient circular wall 120, without trapping anyair pockets. It should be noted, however, that, as already discussed inthe introduction, in reality it is unlikely for the ideal operatingconditions to be present. This means that, in reality, the membraneoften is not flat as it should be and that therefore the air may easilyremain trapped between the pressure sensor 22 and the membrane 12.

The outwardly convex form of the membrane 12 according to the inventioneliminates this risk. In fact, the contact between the membrane 12 andthe end surface 220 takes place gradually, starting from the centre(i.e. the outermost point of the wall 120) and gradually extendingtowards the periphery. In this way, the air is expelled progressivelytowards the outside. Moreover, the convex form of the membrane 12 hasthe effect that, even following relaxation due to ageing of theelastomer or the operating conditions affecting the membrane 12, thewall 120 is able to still perform its function. Such relaxation willresult in the worst of cases in a reduction in the maximum elevation f,but is unlikely to eliminate it entirely and/or reverse the curvature ofthe wall 120 so that it becomes concave.

The function of the membrane 12, as mentioned above also with referenceto the prior art, is to transmit to the sensor 22 the pressure of thefluid and associated variations. From this point of view, the membraneis therefore able to perform its function more effectively, the less italters the progression of the pressure and the more accurately ittransmits it. For this reason, the ideal membrane has always beenconsidered to be that which, having a perfectly flat extension, is notable to generate reaction forces outside of its plane. The membraneaccording to the invention instead, being convex, appears to differmarkedly from this ideal model. The Applicant has instead noted how,surprisingly, the convex membrane according to the invention alsotransfers accurately the pressure progression. Specific tests carriedout in this connection by the Applicant have shown how the behavior ofthe convex membrane according to the invention is, from the point ofview of the transmission of the pressure, absolutely comparable to thatof a flat membrane of the known type. On the other hand, however, theconvex membrane according to the invention introduces substantialimprovements from the point of view of expulsion of the air and reactionto ageing of the elastomer.

The invention also relates to a pressure dome 10 for cooperating with apressure sensor 22, comprising a membrane 12 according to the invention.The pressure dome 10 according to the invention comprises, in a mannerknown per se, a main body 16. The main body 16 defines an inlet 160 andan outlet 161 allowing hydraulic connection to a pipe, for example thepipe of an extra-corporeal circuit.

In accordance with certain embodiments, the main body 16 also defines aseat 162 for stably seating the membrane 12; the seat 162 is inparticular formed so as to receive the rim 124 of the membrane 12.

The main body 16 is preferably produced, in a manner known per se, bymeans of injection-molding of a polymer which is sufficiently rigid andsuitable for contact with the physiological fluids. Polymers suitablefor this type of use may be, for example: polycarbonate (PC),polypropylene (PP), polyethylene (PE), polystyrene (PS), polyvinylchloride (PVC), polyethylene terephthalate (PET), polybutyleneterephthalate (PBT), acrylonitrile-butadiene-styrene (ABS), andcopolyesters.

In accordance with the embodiments shown in FIGS. 2 and 4, the seat 162is defined by an edge 164 and by an inner wall 165. The edge 164 allowsthe membrane 12 to be fixed inside the seat 162. In accordance withthese embodiments, the main body 16 is preferably produced with the edge164 formed as a cylindrical wall (see specifically FIG. 5.a). At thetime of assembly of the main body 16 and the membrane 12, in order toform the complete pressure dome 10, the rim 124 of the membrane 12 ishoused inside the corresponding seat 162 (see specifically FIG. 5.b).Then the edge 164 is folded over so as to press against the rim 124 ofthe membrane 12 and thus keep it inside the corresponding seat 162 (seespecifically FIG. 5.c and FIG. 4).

This system for fixing the membrane 12, achieved by means of deformationof the edge 164, is referred to as beading. Deformation of the edge 164may be achieved, in a manner known per se in relation to the processingof polymers, by means of the application of heat, ultrasound or rotaryfriction. Beading is performed so that the pressure dome is sealed inits entirety, except obviously for the openings which form the inlet 160and the outlet 161. In other words, the joint between the membrane 12and the main body 16 must prevent the physiological liquid, intended tooccupy the pressure dome 10, from entering between the seat 162 and themembrane 12 and therefore escaping outwards.

In accordance with certain embodiments, for example that shown in FIG.4, after the beading operation, the edge 164 is lower than the innerwall 165 which defines, together with the said edge 164, the seat 162.With reference to FIG. 4, the difference in height between the edge 164after the beading operation and the inner wall 165 is indicated by h. Inaccordance with certain embodiments, the height h may range between 0.01and 0.3 mm. In accordance with the embodiment shown in FIG. 4, thedifference h is equal to about 0.15 mm.

The difference in height h between the inner wall 165 and the edge 164ensures a more functional contact between the end wall 220 of thepressure sensor 22 and the membrane 12 of the pressure dome 10. Whencontact occurs between the end wall 220 and the membrane 12, it is thereaction of the wall 165 which defines the contour of the area intendedto transmit the pressure from the pressure dome 10 to the sensor 22. Thecontour defined by the inner wall 165 includes only the active surfacefor transmission of the pressure. This structure therefore ensures thatthe pressure is transmitted in an optimum manner.

In accordance with certain embodiments, the main body 16 comprises asecond outer edge 166. In FIG. 4 this edge is formed as a cylindricalwall, while in FIG. 2 it is shown folded inwards in a manner similar tothat explained above in connection with the edge 164. Unlike the edge164, the second edge 166 is not intended to cooperate with othercomponents of the pressure dome 10. The beading of the second edge 166may, however, be advantageous during cooperation of the pressure dome 10with the pressure sensor 22. The second edge 166, when folded over asshown in FIG. 2, may in fact assist centering of the sensor 22 on themembrane 12 when they are joined together.

FIG. 6 shows a pressure dome 10 according to the invention combined witha multi-use protective element 24. This element is designed to protectthe membrane 12 during all the stages of the life of the pressure dome10, from the time of assembly until its final use in an extra-corporealcircuit. The multi-use protective element 24 is designed to be combinedwith the pressure dome 10, in particular in the position intended toreceive the pressure sensor 22. In this way, the multi-use protectiveelement 24 is able to prevent damage to the membrane resulting fromaccidental knocks or contact which may occur during the logisticaloperations such as handling, packaging, transportation, storage, etc.

The need to protect the membrane arises from the fact that the pressuredome according to the invention must be assembled in its final form in aproduction plant and therefore far from the location of its final use.Assembly, in fact, owing to the beading operation, requires the use ofspecific machinery. On the other hand, assembly of the pressure domeaccording to the prior art may be performed manually, even at the timeof final use, by joining a main body 16 together with a respectivemembrane 12. The membrane 12 according to the prior art may therefore becarefully stored separately until the time of its final use, thusavoiding any damage. The protective element 24 is defined as beingmulti-use because, since it is not intended to come into contact withthe physiological fluids, it may be re-used several times.

According to other embodiments, for example that shown in FIG. 7, themembrane 12 is convex, as described above in connection with the presentinvention, but is fixed to the main body 16 by means of a rigid ring 14,as described in relation to the prior art.

According to these embodiments, the membrane 12 is able to achieve allthe advantages described above with regard to the convex form. Fixing ofthe membrane 12 onto the main body 16 of the pressure dome 10, however,is not achieved by means of beading, but by means of a rigid ring 14suitable for defining a joint with the main body 16. FIG. 7 shows asnap-engaging connection between the ring 14 and the main body 16, butit may also be a screw/female-thread, interference, or similarconnection.

With these embodiments it is possible to combine the advantages of theconvex membrane with the advantage of being able to assemble thepressure dome at the time of final use. In other words, in theembodiment according to FIG. 7, the membrane 12 may be carefully storedseparately and there is therefore no need to preassemble the protectiveelement 24.

According to certain embodiments, the membrane 12 and the ring 14 aremanufactured separately, each one being able to be made for example bymeans of molding of a suitable material. Typically the ring 14 may bemade using one of the polymers listed above with reference to the mainbody 16. The membrane 12, instead, may be advantageously made using athermoplastic elastomer or other elastomers suitable for contact withthe physiological fluids.

According to other embodiments, the membrane 12 and the ring 14 areinstead made by means of two-component injection molding. A single partmade with two different materials is thus obtained, as already describedin connection with the prior art.

In the light of the above description it will be clear to the personskilled in the art how the membrane 12 and the pressure dome 10according to the invention are able to overcome, at least partly, thedisadvantages mentioned in connection with the prior art. In particular,the convex membrane 12 according to the invention is able to reduce to aminimum the possibility of air entering into the joint with the pressuresensor 22.

Moreover, the pressure dome 10 according to the invention, in theembodiments which do not require two-component injection molding, may bemade using simple and reliable technology, while still ensuring the highlevel of quality associated with the pressure domes of the known type.

In connection with the embodiments of the membrane and the pressure domedescribed above, the person skilled in the art, in order to satisfyspecific requirements, may make modifications to and/or replace elementsdescribed with equivalent elements, without thereby departing from thescope of the accompanying claims.

1. Membrane (12) for a pressure dome (10), comprising: a resilientcircular wall (120) suitable for closing one side of said pressure dome(10) so as to define a partition between the inside of the pressure domeand the outside; a circular rim (124) suitable for being joined to amain body (16) of the pressure dome (10); characterized in that theresilient circular wall (120), when there is no difference between thepressures acting respectively on the inner surface (121) and on theouter surface (122), has an outwardly convex form.
 2. Membrane (12)according to claim 1, wherein the resilient circular wall (120), whenthere is no difference in the pressures acting respectively on the innersurface (121) and on the outer surface (122), assumes the form of a cap.3. Membrane (12) according to claim 2, wherein the resilient circularwall (120) has a maximum elevation f, equal to the distance between theoutermost point of the cap and the plane π containing the outer basecircumference of the cap, which is between 1% and 2% of the diameter dof the base circumference of the cap.
 4. Membrane (12) according toclaim 1, wherein the membrane (12) is made as one piece, i.e. whereinthe rim (124) is formed integrally and as one piece with the wall (120).5. Membrane (12) according to claim 1, wherein the membrane (12) is madeof a thermoplastic elastomer.
 6. Membrane (12) according to claim 1,also comprising a rigid ring (14) suitable for defining a joint with themain body (16) of the pressure dome (10).
 7. Pressure dome (10) forco-operating with a pressure sensor (22), comprising a main body (16)which defines an inlet (160) and an outlet (161) suitable for allowinghydraulic connection to a pipe, the pressure dome also comprising amembrane (12) according to claim
 1. 8. Pressure dome (10) according toclaim 7, wherein the main body (16) defines a seat (162) for stablyseating the membrane (12).
 9. Pressure dome (10) according to claim 7,wherein the main body (16) is produced by means of injection-molding ofa polymer which is sufficiently rigid and suitable for contact with thephysiological fluids, said polymer being chosen from the groupcomprising: polycarbonate (PC), polypropylene (PP), polyethylene (PE),polystyrene (PS), polyvinyl chloride (PVC), polyethylene terephthalate(PET), polybutylene terephthalate (PBT), acrylonitrile-butadiene-styrene(ABS), and copolyesters.
 10. Pressure dome (10) according to claim 8,wherein the seat (162) is defined by an edge (164) and by an inner wall(165) and wherein the edge (164) allows fixing of the membrane (12)inside the seat (162) by means of beading.
 11. Pressure dome (10)according to claim 10, wherein the beading operation is performed sothat the pressure dome (10) is sealed in its entirety.
 12. Pressure dome(10) according to claim 10, wherein the edge (164) after the beadingoperation is lower than the inner wall (165).
 13. Pressure dome (10)according to claim 7, wherein the main body (16) comprises a secondouter edge (166) folded towards the inside by means of beading. 14.Pressure dome (10) according to claim 7, also comprising a protectiveelement (24) suitable for being combined with the pressure dome (10) inthe position intended to receive the pressure sensor (22) and designedto protect the membrane (12).
 15. Pressure dome (10) according to claim7, wherein fixing of the membrane (12) onto the main body (16) of thepressure dome (10) is performed by means of the rigid ring (14).